sweets



Jan. 24, 1956 F. M. swEETs 2,732,471

RESONANT HIGH-FREQUENCY HEATING APPLICATOR Filed Feb. 26, 1953 2 Sheets-Sheet l Jan. 24, 1956 F. M. SWEETS 2,732,471

RESONANT HIGH-FREQUENCY HEATING APPLICATOR Filed Feb. 26, 1955 2 Sheets-Sheet 2 Fig.3

UJUIII +l United States Patent RESONANT HIGH-FREQUEN CY HEATING APPLICATOR Foster M. Sweets, Prospect, Ky., assignor, by mesne assignments, to National Cylinder Gas Company, Chicage, 11]., a corporation of Delaware Application February 26, 1953, Serial No. 339,007 9 Claims. (Cl. 219-4055) This invention relates to improvements in resonant high-frequency heating applicators and more particularly to an electrically conductive structure for providing a predominant part of the inductance of the resonant applicator and has for an object the provision of a conductive structure which will permit a maximum mutual inductance coupling between the structure and a coupling loop.

In copending application, Serial No. 138,628, Warren, filed January 14, 1950, now abandoned in favor of Warrens continuation-in-part application, Serial No. 419,633, filed March 26, 1954, there are disclosed certain forms of resonant applicators which comprise an electrically conductive enclosure, in the form of a tunnel, within which are generated both magnetic and electric fields of great intensity. A predominant portion of the inductance of such applicators is provided by at least one electrically conductive structure or fin electrically connected to at least one wall of the enclosure and about which structure the aforesaid magnetic field is produced. The electric field extends between two spaced electrodes, one of which is electrically connected to the free end of the conductive structure. The spaced electrodes provide a substantial portion of the capacitance of the applicator. in order to transfer high-frequency power to or from the applicator or tunnel a coupling loop is positioned within a space encircling the conductive structure.

in order to maintain the voltage between the applicator electrodes substantially constant despite changes in power factor of a load disposed between them, the mutual inductance between the coupling loop and the conductive structure should be supra-optimum. This proposition is more fully discussed in the aforesaid Warren applications wherein it is noted that changes in power factor occur due to changes with heating of the load and in the case of a conveyor-fed applicator to varying numbers of load objects being heated. When the mutual inductance is supra-optimum, i. e., appreciably larger than an optimum value, a substantial change in power factor of the load with a corresponding change in Q of the applicator circuit produces a relatively small change in the electrode voltage, whereas with an infra-optimum coupling the electrode voltage changes very rapidly with change in power factor.

Supra-optimum coupling is readily attained in those cases where the applicator is of great width, for example, when the work being treated is a foam rubber mattress. However, when the applicator is designed specifically for smaller work, and in accordance with the teaching in the aforesaid Warren applications, supra-optimum coupling presents difficult problems.

Methods of providing for supra-optimum coupling in narrow applicators are disclosed in copending application Serial No. 419,072, Warren, filed March 26, 1954, a continuation-in-part of the aforesaid Warren application Serial No. 138,628, and also in a copending application which now has issued as Ellsworth Patent No. 2,712,050. Although such methods now are in use and have performed satisfactorily, the present invention provides the desired supra-optimum coupling in a simpler and more efficacious manner.

In accordance with the present invention, which is an improvement in certain of the conductive structures or fins disclosed in the aforesaid Warren applications Serial Nos. 138,628 and 419,633, and also Warren application Serial No. 419,071, filed March 26, 1954, which also is a continuation-in-part of said application Serial No. 138,628, a resonant high-frequency heating applicator comprised of an enclosure having electrically conductive walls is provided with an electrode within the enclosure in spaced relation to all of said walls and representative of essentially all of the capacitance of the resonant applicator. An electrically conductive structure is electrically connected at opposite ends, respectively, to the electrode and to one of the walls of the applicator to provide essentially all of the inductance of the .resonant applicator. The conductive structure is possessed of a recess extending in directionnormal to the ends thereof and parallel with flow of ,currentltherein and adapted for receiving the side of a coupling loop so that substantially all of the magnetic field within the applicator passes through the coupling loop to provide for supra-optimum mutual inductive coupling.

The invention further resides in features of construction, combination and arrangement hereinafter described and claimed.

For a more complete understanding of the invention and for illustration of various modifications thereof, reference is made to the accompanying drawings, in which:

Fig. 1 is a perspective view schematically portraying a resonant high-frequency heating applicator embodying the present invention;

Fig. 2 is an enlarged perspective view of the conductive structure and the electrode shown in Fig. 1;

Fig. 2A is a perspective view disclosing a modification of the conductive structure of Fig.2;

Pig. 3 is an enlarged cross-sectional view, in a vertical plane defined by the lines 33 of Fig. 1, with a portion of structure 15 in elevation, and disclosing one of many relations between the coupling loop and the conductive structure, and also schematically illustrating a typical oscillator used in conjunction with a resonant applicator; and

Fig. 4 is a perspective view of a conductive structure and electrode embodying the broad concept of the present invention.

Referring now to the drawings, wherein like reference characters designate. the same parts throughout the several views, and more particularly. toFig. 1, there is disclosed a resonant high-frequency heating applicator 10 comprised of electrically conductive walls 11 through 14 and having mounted therein apparatus providing for substantially all the inductance and capacitance of the applicator. Although the applicator as shown is open at both ends, and may be so used, it is preferably a more complete enclosure to minimize leakage of the intense magnetic and electric fields existing therein.

.In the illustrated embodiment, substantially all the inductance for theapplicator is providedby an electrically conductive structure 15 which may be secured or otherwise electrically connected to one of the conductive walls of the applicator, usually the top wall 11, and depends therefrom in spaced relation to the other walls. The length of the structure 15 from the connected end to the depending end is small as compared to the quarterwavelength of the. frequency of current passing .therethrough. The.structure may be treated as a lumped inductance in which the current gradient is negligible in avoidance of standing waves.

The heating of a load subjected to theaction of the applicator is produced within an electric field generated between an electrode 16 and the wall 13. Although not specifically disclosed in Fig. 1, a metallic conveyor may serve as the, lower heating electrode so as to provide for continuous processing of work. The upper electrode 16, which may consist of a plate of" electrically conductive material such as copper, and preferably rectangular in shape though not necessarily so, is secured or otherwise electrically connected to the free end of the conductive structure 15 at a portion intermediate the ends thereof.

In the particular embodiment illustrated the shortest distance from any common point of the structure 15 and electrode 16 to the adjacent edges of the electrode is small as compared to a quarter-wavelength at the frequency of voltage on the electrode and, therefore, the electrode may be treated as a lumped capacitance. With such arrangement the voltage at all points on the electrode will by copper piping which serves to conduct cooling liquid 'for the oscillator tube, a construction claimed in copending Warren application Serial No. 419,074,. filed March 26, 1954.

A suitable oscillator circuit is shown in Fig. 3 so that the purposes and advantages of the invention may be better understood. The anode of the tube 18 is connected to one terminal of the loop 17 whose other terminal is connected to a wallet the applicator grounded at a. A direct-current source of 'high voltage 13+, B- is connected between ground and the cathode of the tube 18, the positive end of that terminal being grounded as indicated. The grid of tube 18 is connected to the tunnel electrode 16 through adjustable capacitor 19. The capacitor 19 and acapacitor 20, in whole or in part provided by the efiective input capacitance of tube 18, provides a capacitive potential divider which, as explained in the aforesaid Warren applications Serial Nos. 138,628

and 419,633, is utilized in the automatic stabilization of the grid voltage. A direct current path between the grid and cathode of tube 18 is provided by radio frequency choke 21 and the grid leak resistor 22.

As set forthabove, it is desirable to maintain the voltage at the electrode 15 substantially constant despite variations in power factor within the applicator as produced by variations in load. To do so requiressupra-optimum coupling between the coupling loop '17 and the conductive structure 15. This condition is always attained when substantially all of: the magnetic field circulating about'the conductive structure is coupled with or: cut by the loop 17 In some instances it is necessary to reduce the electrode voltage to a required lower value. Supra-optimum coupling aids in securing such voltagereduction. Limitations as to the percentage of the magnetic field'coupled by the loop are presented by the proximity between a leading side of the loop 17 with respect to the structure 15 and the danger of flash-over between the loop and the electrode 16. That the intensity between the electrode16 and the loop 17 may in someinstances be relatively large it becomes apparent when it is realized that'the voltage difference between the loop and the electrodeimay be as high as 32,000 volts, or even more, which is at radio frequency.

In accordance with the present invention, the conductive structure 15 is provided with a recess 25 defined by a center section 26 offset from the plane of the main body of the conductive structure 15 as defined by plates 15:: and 15b. The configuration of the conductive structure 15 as shown in Fig. 2 may be formed of a single solid sheet of copper or other'rnaterial having similar characteristics and bentsothat the center section 26 takes on a substantially U-sha'ped configuration. The recess formed thereby permits the coupling loop 17 to be positioned in such a manner as to provide a degree of coupling heretofore impossible without necessarily deforming the magnetic field encircling the conductive structure as disclosed in the aforesaid Ellsworth patent. It will be observed that the conductive structure 15 is continuous so that even though the coupling loop 17 is positioned beyond the plane of the main body of the structure 15 the loop will only be affected by flux traveling in a single direction. Although the coupling loop 17 is shown to be positioned within the recess, it will be understood that in some instances it' will be sufficient for the proper operation of the applicator to position the leading end 17a of the coupling loop within close proximity to the opening of the recess and substantially within the plane defined by the plates 15a and 15b of the conductive structure 15.

Although the conductive structure 15 has been set forth above as being formed of a single sheet of material, it will be understood that the particular configuration illustrated in Fig. 2 may be constructed of separate sheets of material joined together in any well known manner, such as by welding or riveting. It is to be further understood that although'the configuration-of the recess is illustrated as rectangular or substantially U-shaped, the particular configuration defining the recess is relatively unimportant so long as the recess is formed by any suitable member which is a continuous or integral part of the conductive structure 15. The integral aspect of construction is necessary to prevent the loop 17, when positioned within the recess, from being cut by flux paths of opposite direction which obviously would be undesirable.

In illustration of the many modifications possible within the scope of the present invention, reference is made to Fig. 2-A wherein a conductive structure 30, functioning in like manner to that of the conductive structure or fin 15 of Fig. 2, is illustrated as being comprised of a single sheet of material having an arcuate cross section and secured intermediate the ends of an electrode 16a. The concavity thus produced defines the recess 25a for receiving the leading end 17a of the coupling loop 17.

-An advantage in the construction illustrated in Fig. 2-A

is the reduction in number of sharp edges presented to the circulating magnetic field. The presence of sharp 'edges may induce a concentration of current at such points, resulting in excessive heating. It is now apparent from the illustration of Figs. 2 and 2-A that the center section 26 shown in Fig. 2, which is substantially U- shaped, may be replaced or supplanted by a section having an arcuate cross section of the type illustrated in Fig. 2-A.

Afurther example of a construction which may provide for supra-optimum coupling is illustrated in Fig. 4.

It is to be understood, however, that the structure there 'shown is not to be considered as my invention, but is described and specifically claimed in copending U. S. application Serial No. 339,054, filed February 26, 1953, by Ellsworth.

-In the construction shown, a structure 40 formed of electrically conductive sheet metal includes a recess 41. The conductive structure comprises closed hollow columns 40a and 40b interconnected by a web section 40c which lies tangentially with the columns.- It will be observed that the end surfaces of the conductive structure 40 are rounded was to provide for an even distri- -'bu tion of current throughout the structure. The fin or conductive structure 40 is secured to a plate 42 which may represent part of the wall structure 11 shown in Fig. 3. The supporting structure or plate 42 is shown to have rounded surfaces for the same purpose aforesaid.

The other end of the fin or conducting structure 40 is securely mounted to an electrode 43 byway of a rectangular plate 44 secured to an electrode reinforcing member 45. The bottom. of the electrode is comprised lot a sheet of conductive material 46 having its outer portions bent up and around so as to present an arcuate surface about the periphery thereof in order to reduce to a minimum corona effects inherent in conductive surfaces having sharp edges. Like the apparatus of my invention as disclosed in Figs. 1-3, the conductive structure 40 may be considered a lumped inductance providing substantially all the inductance of the applicator, and the electrode 43 providing substantially all the capacitance for the applicator and also considered as a lumped capacitance.

It is, therefore, to be understood that further modifications and variations of the present invention are possible in the light of the above teachings and that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A high-frequency applicator comprising a pair of electrodes supported in spaced relation one from the other, electrically conductive inductance structure of substantial length and width electrically connected at one L end to one of said electrodes and extending away from said one electrode, a housing enclosing said structure and the space between said electrodes, means including walls of said housing electrically interconnecting the other end of said conductive structure with the other of said electrodes, said conductive structure having an intermediate portion spaced at greater distance from an adjacent wall of said housing than the edge portions of said conductive structure to form a recessed area extending throughout a major portion of the length of the inductance structure in a direction parallel to current flow therethrough for enlargement or" a portion of the space through which extends the magnetic field encircling said conductive structure, and a coupling loop extending from said adjacent wall and dimensioned within said enlarged space for traverse of said loop by a large portion of the magnetic field encircling said inductive structure.

2. A high-frequency applicator comprising a pair of electrodes supported in spaced relation one from the other, electrically conductive inductance structure of substantial length and width electrically connected at one end to one of said electrodes and extending away from said one electrode, a housing enclosing said structure and the space between said electrodes, means including walls of said housing electrically interconnecting the other end of said conductive structure with the other of said electrodes, said conductive structure having intermediate the opposite edges thereof a recess facing toward an opposed wall of the housing and which recess is so formed that portions of the conductive structure which lie at the bottom of the recess are at a substantially greater disance from said opposed wall than are portions of the conductive structure which lie on opposite sides of the recess, and a coupling loop dimensioned and positioned so as to be traversed by a large proportion of the magnetic flux encircling said conductive structure, said loop having a relatively large area and being disposed in the space subtended by said recess between said conductive structure and said opposed wall, and said loop extending within such close proximity to the opening of said recess that the distance between said conductive structure and the adjacent portion of said loop is substantially equal to or less than the depth of said recess.

3. The high-frequency applicator of claim 2 wherein the conductive inductance structure comprises spaced apart aligned sections, and an intermediate section joining said sections to present an unbroken surface and substanlially U-shaped in cross section to provide the recess for the coupling loop.

4. The high-frequency applicator of claim 2 wherein the recess extends from one end of said conductive inductance structure to the other end thereof.

5. The high-frequency heating applicator of claim 2 wherein the conductive inductance structure is comprised of a pair of symmetrical fiat sheet metal portions posi- 6 tioned in a common plane and joined by a substantially U-shaped portion providing the recess.

6. The high-frequency heating applicator of claim 2 wherein the conductive inductance structure is comprised of a pair of fiat sheet metal portions positioned in a common plane and joined by another portion having a concavity therein extending parallel with the flow of current in the structure to provide the recess.

7. A high-frequency heating applicator comprising a pair of electrodes supported in spaced relation one from the other, conductive structure electrically connected at one end to one of said electrodes and extending away from said one electrode, a housing enclosing said structure and the space between said electrodes, means including walls of said housing electrically interconnecting the other end of said conductive structure with the other of said electrodes, said conductive structure comprised of spaced apart aligned sections, and an intermediate section joined said sections to present an unbroken surface, said intermediate section being substantially U-shaped in crosssection to provide a recess extending in direction parallel to the current flow through said structure, the recess extending from one end of said conductive structure to the other end thereof, and a coupling loop having a side positioned within the recess.

8. An applicator for high-frequency electric heating of dielectric materials comprising a reentrant cavity resonator having an electrically conductive housing with reentrant structure projecting into the interior thereof, inductance structure within the housing and comprising at least one inducive element of substantial length and Width projecting into the interior of the housing and constituting at least part of said reentrant structure, spaced electrode structures cooperative to provide electric field space within the housing, one of which electrode structures is disposed at the inwardly projecting end of said inductive element in spaced relation to the wall structure of the housing and electrically connected with the wall structure of the housing through said inductive element, said wall structure providing a path completing a resonant circuit which includes said inductance structure and said electrode structures and the frequency of which is predominantly deter mined by the inductance of said inductance structure and the capacitance between said electrode structures, said housing serving as a shield to confine the electric field produced between said electrode structures and the mag netic field encircling said inductive element, said inductive element between opposite edges thereof having a recess extending in direction parallel with current flow thererhrough to provide a greater spacing from adjacent wall structure of said housing than at said opposite edges, and a power-transfer coupling loop disposed within the housing in said region of greater spacing and dimensioned for traverse therethrough of the major portion of said magnetic field encircling said inductive element.

9. An applicator for heating dielectric materials com prising an electrically conductive housing, inductance structure therein comprising a fin inductor of substantial length and width projecting into the interior of said housing, spaced electrode structures cooperative to provide an electric field space within the housing, one of said electrode structures being disposed at the inwardly projecting end of said fin inductor in spaced relation to walls of the housing and electrically connected with wall structure of the housing through said fin inductor, said wall structure providing a path completing a resonant circuit which includes said inductance structure and said electrode structures and the frequency of which is predominantly determined by the inductance of said inductance structure and the capacitance between said electrode structures, said fin inductor between its opposite edges having a recess extending in a direction parallel to current how therethrough to provide increased spacing from the back of said recess to the opposing wall of said housing, a coupling loop disposed within said housing and extending from said opposing wall toward and having at least a portion thereof extending at least to the mouth of said recess to be threaded by substantially all of the magnetic field encircling said fin inductor to provide supraoptimum mutual inductance between said applicator and said loop.

, References Cited inthe file of this patent UNITED STATES PATENTS Goldstine' Sept. 24, 1940 D'akin' Apr. 18, 1950 Zottu May 9, 1950 

